Keeping mold at bay

The good news for builders is that most mold damage in homes and occupied commercial structures comes from flooding or neglect. The notorious Ballard case in Texas unfolded over many years of plumbing leaks that went unaddressed. Many others involve floods from storms. These cases are often covered by the owner’s insurance and don’t usually involve the builder.
The bad news is that some mold problems are due to building defects, and builders can be on the hook even for problems they did not cause. And virtually all liability insurance now severely limits coverage for mold claims.
Back to the good news: the techniques that can ward off both mold and lawsuits generally won’t take you far from good building design and practices. There are a few extra precautions to take and procedures to follow, and some projects may call for special mold-resistant materials. According to one study, mold protection adds $500-1000 to the cost of a new home. But most are things builders should be doing anyway.
Preventing mold is mostly about managing moisture, which can cause rot and other problems unrelated to mold. “We hit on this again and again,” says Jay Willis of Engineering Consulting Services in Roanoke, Va., who does mold seminars and inspections. “It’s all about controlling the moisture.”
Keeping moisture out of the structure involves virtually every phase of construction — including the basic design elements. Before you begin a project, assess its risk for mold. If the building is in a high-humidity area with a tricky wall system such as EIFS, consider passing or requesting special contract terms. A stucco-sided home with small eaves will have more trouble keeping water out. Take precautions in flood-prone areas.
Generals should choose roofing, HVAC, siding, and plumbing subcontractors with care; their work is most likely to cause moisture problems, and you could be liable as well. Finally, train your crews in mold recognition and procedures. Consider appointing a company “point person” for mold issues.
An array of design details have been developed to minimize moisture intrusion. Basement walls should be backfilled for drainage, and insulated and waterproofed if necessary. Prevent moisture from wicking up through concrete into framing with appropriate flashing. Exposed soil in crawl spaces should be encapsulated with a moisture barrier.
Wall details are particularly critical. Proper placement of moisture and air barriers (housewrap) and vapor barriers is crucial to limiting condensation inside walls. Creating a “continuous drainage plane” behind the exterior finish is crucial to stopping weather-related moisture. The flashing and moisture barrier details differ for brick, stucco, and wood or vinyl siding. APA—The Engineered Wood Association has published a set of brochures, titled “Build a Better House,” showing non-proprietary details for foundations, roofs, and walls. They are available at, or by calling (253) 565-7265.
The five most common mistakes leading to water intrusion, according to the APA’s Greg Bates, are:
1. Improperly flashed doors and windows.
2. No exterior drainage plane.
3. Reversed shingling of housewrap flashing, or other moisture barriers.
4. Improper grading to keep water away from foundation walls.
5. No kick-out flashing to keep rainwater from running behind the siding and get trapped in the wall.
Sometimes bad practices conspire with bad luck to create problems. “We had a case where housewrap wasn’t lapped properly,” says Willis, “and when they bricked up the house, one fell behind and tore the wrap. Moisture had a pathway right into the wall.”
Many builders suspect that the tight building techniques developed over the past few decades (housewrap, vapor barriers, and other products and procedures that limit air and vapor movement) are to blame for more mold outbreaks because they don’t allow walls to dry out as easily. The NAHB Research Center disputes the notion, noting that most mold problems arise from plumbing and weather leaks. But the truth is that tight construction means a smaller margin for mistakes.
In addressing condensation issues, an air and moisture barrier is of primary importance, since air movement across a wall can carry far more moisture than vapor diffusion, which is controlled by vapor barriers. Proper warm-side location of vapor barriers is important, but more critical is avoiding vapor barriers on both sides of a wall. Since wallpaper, some paints, and even OSB can sometimes act as a vapor barrier, this requires more attention than builders have given it.
Many builders view mold as just one component in the concern for indoor air quality. Tight construction means lower air changes per hour; add mold, rising allergy and asthma rates, and indoor pollutant concerns, and you have a convergence of factors arguing for improving mechanical ventilation systems. New and improved air-to-air exchangers, HEPA filters, whole-house vacuums, and other mechanical ventilation systems can improve air quality and ensure low indoor humidity. Systems such as automatic bathroom fans can head off mold problems no matter how careless the inhabitants.
Jobsite procedures can also lower the incidence of mold in a home’s early years. Keep lumber, wallboard, and other building materials dry. This requires proper storage methods; better yet, schedule deliveries to arrive just in time.
Before closing up walls, check framing lumber for moisture content. According to the Western Wood Products Association, lumber does not support mold when it dries to below 20 percent moisture, which it should do naturally during framing in most circumstances. Nevertheless, some builders go armed with moisture meters and won’t close up walls until the lumber is between 10 and 14 percent moisture.
Contractors are also beginning to inspect all framing, especially sill plates, and spray suspect spots with 10 percent bleach solutions or other mold-killing products. For the very nervous, areas prone to moisture can be sprayed with mold-resistant paints. But enclosed areas shouldn’t really be prone to moisture to begin with.
Many builders insist on air pressure-testing plumbing pipes for several days before closing up walls.
Mold concerns have prompted some builders to look at their building materials. “I won’t use OSB on any vertical wall,” says Don Geddes of Michigan Valley Homes. Others have taken to using kiln-dried lumber for all framing.
As for “mold-resistant” products, the market is growing fast but still limited. (One discussion of substitutes for wallboard suggested precast or epoxy-impregnated concretes — obviously unrealistic alternatives for most builders.) There are a variety of ASTM standards for mold resistance — ASTM D3273, ASTM C1338 for insulation, ASTM D2020 for paper and paperboard, etc. — but the operative word is “resistance.” Products with added chemical biocides won’t stand up to mold for long with enough moisture around.
After construction, builders should ensure that home and building owners are informed on mold issues, particularly on the importance of maintenance and keeping humidity low. The NAHB Research Center has a useful tip sheet available at, and its Toolbase Technotes also has valuable materials (available online at or by calling (301) 249-4000.)
Mold can grow within 48 hours on wet building materials. If a water leak is found in a building you’re involved with, document it but don’t stop to call your lawyer: just clean it up. Vent the walls, begin de-humidification, and take out the carpets.
This advice comes from Kyle Dotson, a Houston-based Certified Industrial Hygienist. “You’ll spend a few hundred cleaning up a small outbreak. If you wait to figure out who’s responsible, it will quickly become a $20,000 problem.” A contractor’s quick response will go a long way in creating goodwill with an owner and heading off a lawsuit.
Contractors themselves can clean up minor mold outbreaks, less than 10 sq. ft. Outbreaks larger than 30 sq. ft. should probably be handled by a company licensed for mold remediation. If legal or health concerns arise, Dotson recommends using a different company — preferably a Certified Industrial Hygienist — to perform post-remediation testing. Remediators who perform their own testing have an unavoidable conflict of interest.

Related Posts:

  • No Related Posts